Window films are sought for their heat rejecting property while allowing visible light to come in. Therefore, the clarity and the infrared rejection ability are two important considerations for window film applications. In order to reject most of the heat, existing technology uses metal/ITO multilayer stacks or tinted multilayer film technology. However, this reduces the transparency of window films for automobile and building applications. In a typical building, lighting and cooling occupies about 65% of the electricity cost. With the rising cost of cement and steel, coated glass with high natural lighting and high IR rejection property is an attractive alternative for a greener building. Such property is highly desirable in window coating applications used for building architecture and automobiles which can greatly reduce the transmission of heat from the exterior to enter the air-conditioned area. This keeps the interior of the buildings and automobiles cool and decreases the high dependency on air-conditioning, hence reduces the electricity used. In addition, with a good transparency in the visible range, natural light from the exterior can also enter which reduces the need of artificial lights during day time as compared to a highly tinted glass. Many conventional multilayer window films utilize transparent conductive oxide (TCO) layers. The problem of using TCO is that there are stability issues with the TCO when being used in conjunction with transition metal layers.
Furthermore, indium tin oxide (ITO), which comprises of about 90% In2O3 and about 10% SnO2, has been the primary transparent conductive oxides (TCO) for display technology, photovoltaics and optoelectronics applications. However, due to limited indium supply, the price of indium has been increasing. Therefore, indium is a high cost raw material. Currently, most commercial multilayer structures use indium oxide or an ITO film. Alternative transparent conductive oxides such as doped zinc oxide are not easily optimized for producing a good quality film. There is therefore a need for coatings for glass that avoid the use of costly ITO and TCO.
For Silver-based multilayer coating, for example, there is a critical issue faced by the heat rejection coating industry which is the oxidation of the transition metal, especially Silver, which causes the degradation of the overall performance of these heat rejection coatings. There is therefore a need for multilayer coating, which avoids oxidation of the transition metal.
Further, damage to organic materials by UV light is a well-known issue. Common organic materials like plastics, polymers and wood will experience a rapid photolytic and photo-oxidative reaction, which will result in their photo-degradation.
Existing window films use a large number of layers. This means that the more layers they have, the more complicated their fabrication process will be. Some existing technologies utilize up to 100 layers.
There is therefore a need for a multilayer coating, which can be used to increase the heat rejection property of the film while maintaining high visible light transparency. There is also a need for a multilayer coating with a minimum amount of layers for ease of fabrication and materials cost. Further there is the need for a multilayer coating without using a TCO layer.
Accordingly, there is a need to provide a multilayer coating that overcomes, or at least ameliorates, one or more of the disadvantages described above.